This invention lies in the field of screening particulate material, non-limiting examples are screens which are used extensively in grading of ores and other materials in mining and process operations. In addition to woven wire screens and perforated sheet and plate screens, screens made up of a plurality of screening panels have attained wide application. Such screening panels, for example, are manufactured by casting a suitable grade of polyurethane and the panels are mounted contiguously in a tiling fashion on a screening frame.
The fastening of such screening panels onto a frame has received attention in engineering design and been an important factor in the successful application of moulded polyurethane screen panels. Many designs have been developed, merely as examples may be mentioned fastenings shown in South African patent 73/5815 granted to Screenex Wire Weaving Manufacturers (Pty) Ltd and South African patent 80/4039 granted to Steinhaus GmbH. Many variations of such designs have been developed over the intervening years, a feature of the fastenings used up to now being that they are made of the same polymeric material as the screen panels which they hold.
Vibratory frequencies and amplitudes specified in current practice have been selected so that acceleration of the panels at the fastenings does not exceed a generally accepted limit. This limit is about 4.5 G (four point five times the acceleration due to gravity). Beyond this limit experience has shown that the fastenings become unreliable and/or tend to fail prematurely while in service.
Modem practice requires accelerations in excess of the limit mentioned.
Another system used, the so-called clipper system, does have better holding capacity at high accelerations, but is not always appropriate, depending on various considerations applicable to a particular installation, customer""s requirements, and the like. Other systems available are subject to wear resulting from a tendency of material being screened seeping into crevices between the fixing parts and the frame causing the frame fixing locations to wear out. The wear reaches a stage where the fastening pin system fails. For example, the hole in the frame in which the fastening is secured becomes oversize due to wear, until the fastening fails. The material being screened in many cases becomes an unfortunately very effective grinding medium acting on the frame.
Thus the screening frame is also an important component in the fastening and acceleration problems which must be addressed in current practice. In current practice the screening frame may either be a frame made up for fixing to a screening machine or it may be a frame existing as part of the screening machine. Such screens generally are a structural fabrication of elongate structural members, usually extending in longitudinal and transverse directions. They form a grid upon the members of which screen panels can be mounted and secured. Structural members commonly used include girders, joists and other beams which may be of I section, angle section, channel section or square tube section, for example. The girders are in general of larger section and are spanned by joists of smaller section. All the sections used provide a web which has a depth sufficient to carry the vertical reciprocating loads and a flange which presents a horizontal surface to support the panels which are carried by the screen frame and to fix them to the frame.
In accordance with the present invention, there is provided a fastener for a screening panel, which fastener comprises a combination structure including a metal tensile component and a polymeric holding component which includes a ledge for fastening a panel.
The ledge may be in the form of a shoulder, bush, ridge, land or like structure, which is adapted to fasten a panel down, generally against a screen frame. The screen frame may be a part of a screening machine, or may be made up for a particular application, to be fastened to a screening machine. The ledge must be of sufficient bearing area on the panel, to effectively fasten the panel, during use. If the area is too small, the shearing stresses in the panel material rise to levels at which localised heating develops and the molecular structure of the polymer tends to degrade, leading to failure after a period. With sufficient area, in accordance with this invention, the holding component adequately holds the panel down for more than the service life of the panel.
Although the fastener according to this invention could be used with a conventional frame, the present invention also provides a stringer for preferred use as a joist in a screen frame, the stringer comprising a strip of cross section having a narrow width and depth sufficient to carry vertical reciprocating loads, presenting neither a top nor a bottom web, but a plurality of spaced apart platforms, each platform presenting a fastening formation for a panel and support area around or adjacent the fastening formation, for a panel.
For example, each platform can comprise a disc having a hole in its centre, the hole providing a fastening formation and the surround of the disc a support area for the panel around the hole.
It is desirable that the platform is positioned above or coinciding with the centroid of the stringer section, so as to avoid exerting any eccentric loads on the stringer. If a fastening formation in the form of a hole is then located immediately above the centroid of the stringer, then immediately underneath the hole a recess can be provided in the stringer to accommodate a fastener which co-acts with the hole to fasten the panel, for example, a bolt head of a bolt.
Preferably the upper support surfaces of the platforms is flush with the upper surface of the stringer, so that these surfaces all provide support for panels mounted on the stringer.
Preferably the fastener and the stringer of this invention are used together in order to obtain the maximum advantages of the invention.
In accordance with a preferred embodiment of the invention, screening panels are provided with co-acting formations for co-acting with the fasteners, including in particular, the ledges of the fasteners. Preferably that part of the formation for co-acting with the ledge of each fastener is located at an intermediate position in the depth of the panel.
The provision of fasteners in accordance with this invention together with panels having co-acting formations thus also falls within the scope of this invention.
In a preferred application of the invention, those screening panels which are mounted contiguous with adjacent panels are provided with these formations at their edges; this allows the fasteners to each simultaneously fasten two contiguous panels of this kind.
The metal tensile component conveniently in most applications comprises a bolt and nut. A lock washer or locking nut is advisable. The polymeric holding component is preferably a stepped bush through the hole of which the bolt is passed, the step of the bush providing the required ledge for fastening the panel.
In a preferred embodiment, the fastener structure further includes a cover for an exposed portion of the metal tensile component. Where an upper end of the tensile component would otherwise be exposed, it is preferably covered by the cover. The reason is to avoid, as far as possible, exposure of this end to abrasive material being screened and the ingress of material into interstices between the metal tensile component and the polymeric holding component and between the fastener combination structure and the panel and/or the screening frame to which the panel is fastened. For example, the nut and threaded end of the bolt may be uppermost, this is best covered with the cover.
Preferably, the cover is provided with a skirt adapted to be fitted onto the tensile member with the skirt providing an improved barrier to ingress of material being screened into the interstices mentioned above.
Where a panel which has co-acting formations for the fastener is provided, preferably the formations include also a portion for co-acting with the skirt. Preferably these formations allow the skirt to enter partially into the depth of the panel.
In accordance with a preferred embodiment of the invention, the fastener is provided in combination with a toggle or butterfly near an end of the metal tensile component, for engaging a blind hole in a screening frame. A blind hole may arise where the frame uses, for example, square tube members, or other structural members where the tensile member must be connected without access to both sides of the member. The toggle may comprise a strip or plate with the tensile member passing through an offset hole, the plate narrow enough to be passed through a hole in the screening frame member when misaligned, but long enough to be held when aligned. The hole in the screening panel must be of larger diameter than would be necessary for the shaft of the tensile member, e.g. a bolt, to pass through in normal manner. This enlarged hole is large enough to allow the tensile member and toggle to pass through, suitable angled to minimise their effective size. Preferably the polymeric holding component is provided with a further step which fits into the enlarged hole provided in the screen frame, thus locating the tensile member and toggle in a holding position with enhanced security.